ABSTRACT

There is increasing, but largely indirect, evidence pointing to an effect of commensal gut microbiota on the central nervous system (CNS). However, it is unknown whether lactic acid bacteria such as Lactobacillus rhamnosus could have a direct effect on neurotransmitter receptors in the CNS in normal, healthy animals. GABA is the main CNS inhibitory neurotransmitter and is significantly involved in regulating many physiological and psychological processes. Alterations in central GABA receptor expression are implicated in the pathogenesis of anxiety and depression, which are highly comorbid with functional bowel disorders. In this work, we show that chronic treatment with L. rhamnosus (JB-1) induced region-dependent alterations in GABAB1b mRNA in the brain with increases in cortical regions (cingulate and prelimbic) and concomitant reductions in expression in the hippocampus, amygdala, and locus coeruleus, in comparison with control-fed mice. In addition, L. rhamnosus(JB-1) reduced GABAAα2 mRNA expression in the prefrontal cortex and amygdala, but increased GABAAα2 in the hippocampus. Importantly, L. rhamnosus (JB-1) reduced stress-induced corticosterone and anxiety- and depression-related behavior. Moreover, the neurochemical and behavioral effects were not found in vagotomized mice, identifying the vagus as a major modulatory constitutive communication pathway between the bacteria exposed to the gut and the brain. Together, these findings highlight the important role of bacteria in the bidirectional communication of the gut–brain axis and suggest that certain organisms may prove to be useful therapeutic adjuncts in stress-related disorders such as anxiety and depression.

There is increasing evidence suggesting an interaction between the intestinal microbiota, the gut, and the central nervous system (CNS) in what is recognized as the microbiome–gut–brain axis (1–4). Studies in rodents have implicated dysregulation of this axis in functional bowel disorders, including irritable bowel syndrome. Indeed, visceral perception in rodents can be affected by alterations in gut microbiota (5). Moreover, it has been shown that the absence and/or modification of the gut microflora in mice affects the hypothalamic–pituitary–adrenal (HPA) axis response to stress (6, 7) and anxiety behavior (8, 9), which is important given the high comorbidity between functional gastrointestinal disorders and stress-related psychiatric disorders, such as anxiety and depression (10). In addition, pathogenic bacteria in rodents can induce anxiety-like behaviors, which are mediated via vagal afferents (9, 11).

Accumulating clinical evidence suggests that probiotics can modulate the stress response and improve mood and anxiety symptoms in patients with chronic fatigue and irritable bowel syndrome (15, 16). One such organism isLactobacillus rhamnosus (JB-1), which has been demonstrated to modulate the immune system because it prevents the induction of IL-8 by TNF-α in human colon epithelial cell lines (T84 and HT-29) (17) and modulates inflammation through the generation of regulatory T cells (18). Moreover, it inhibits the cardio–autonomic response to colorectal distension (CRD) in rats (19), reduces CRD-induced dorsal root ganglia excitability (20), and affects small intestine motility (21).

Furthermore, in this study we observed that L. rhamnosus (JB-1) administration reduces the stress-induced elevation in corticosterone, suggesting that the impact of the Lactobacillus on the CNS has an important effect at a physiological level. Alterations in the HPA axis have been linked to the development of mood disorders and have been shown to affect the composition of the microbiota in rodents (26). Our data are in line with previous studies showing that subchronic or chronic treatment with antidepressants can prevent forced swim stress-induced increases in plasma corticosterone in both mice and rats (27). Moreover, it has been shown that alterations in HPA axis modulation can be reversed by treatment with Lactobacillus andBifidobacterium (28, 29).

The vagus nerve plays a major role in communicating changes in the gastrointestinal tract to the CNS (3). In the present study, Vx prevented the anxiolytic and antidepressant effects of L. rhamnosus (JB-1) and also the changes in GABAAα2 and GABAAα1 mRNAs in the amygdala (SI Materials and Methods), as well as GABAAα1mRNA in the hippocampus.

Moreover, vagus nerve stimulation has been described as a successful approach to treat some (44), but not all (45), patients with treatment-resistant depression, which further suggests the importance of the vagus nerve in the modulation of behavior.